UNITED NATIONS — Leaders of humanitarian aid organizations have, over the last few months, repeatedly implored the warring parties in Syria and their backers not to use the delivery of food and medicine as a bargaining chip in their negotiations about war and peace.

But the leaders of the world’s most powerful countries did just that on Thursday night, striking a deal over access to aid during diplomatic negotiations in Munich. The very groups that had been begging for relief to besieged towns now welcomed the promised relief with extreme caution, describing it as a half-step that only underscored how bread, blankets, and even vaccines for children, have become politicized in the five-year-old civil war.

Access to aid is enshrined under international law. But in this conflict, it has been repeatedly used as leverage on the battlefield and at the negotiating table. The United Nations has principally accused the Syrian government, but also the Islamic State and some armed opposition groups, of putting up blockades to the delivery of food and lifesaving medicines to civilians.

The United Nations secretary general, Ban Ki-moon, has warned that those who use starvation as a weapon are committing war crimes.

As if to underscore a blatant disregard for the neutrality of aid, Russia boasted in announcing the Munich deal that it would use its military planes to airdrop humanitarian relief.

“While we welcome any effort to improve the lives of Syrians in such desperate need of humanitarian assistance, the conflation of humanitarian issues with political agendas poses a serious threat to the independence, neutrality and impartiality of humanitarian action,” said Mathieu Rouquette, an aid worker with Mercy Corps, who belongs to a consortium of international agencies that works in both government and rebel-held areas of Syria.

Where and whom aid convoys can reach, Mr. Rouquette added, should not be decided by diplomats in closed rooms as “a tool in the palette of political options.”

Mercy Corps and other aid groups have been caught in a bind as the sieges have intensified. They have condemned the use of aid access as a political tool — but remained desperate for the world powers to do whatever they can to lift the crippling blockades.

So far, Moscow has limited its aid drops to towns controlled by its ally, President Bashar al-Assad. Whether its airdrops in the future will include territory controlled by the opposition — where many aid groups supported by Western donors operate — remains to be seen.

The International Committee of the Red Cross, which has repeatedly called on all sides to respect international law and lift the blockades, cautiously praised the Munich deal, but not without a scold.

“It is high time the world powers and the warring parties start putting Syrians first and their own interests second,” said Robert Mardini, the group’s regional director for the Middle East. “Those who hold Syria’s present and future in their hands must pause for a moment and think of the appalling suffering that prevails all over the country and think of how they can help us restore a little hope.”

Only last week, during an international donors conference on Syria, the president of the International Committee of the Red Cross, Peter Maurer, posted on Twitter: “#Humanitarian aid is becoming a bargaining chip in political negotiations. We don’t accept this.”

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For Syrians fleeing airstrikes in Aleppo Province and hoping to find refuge in Turkey, it remains to be seen what the recent cease-fire agreement means. Most of them are still stuck at the border.CreditCreditBulent Kilic/Agence France-Presse — Getty Images

But that is exactly what happened.

The deal on aid access was announced late on Thursday in Munich by Secretary of State John Kerry and the Russian foreign minister, Sergey V. Lavrov, alongside what they described as a temporary and limited “cessation of hostilities” to allow aid convoys to reach those living under siege.

The “pause,” as Mr. Kerry carefully called it, is to start within a week. And it allows bombing to continue so long as it is aimed at the Islamic State, also known as ISIS or ISIL, or the Al Qaeda affiliate known as the Nusra Front, both listed as terrorist groups by the United Nations Security Council.

“We must ask: Why wait one more week before the fighting stops?” read a statement laced with irritation issued by the chief executive of Mercy Corps, Neal Keny-Guyer. “It’s time to end the fighting. It’s time to end the bombing. It’s time to end the war. Now.”

David Miliband, the former British foreign secretary and current president of the International Rescue Committee, said flatly that the Munich deal waits too long and lacks details. Health centers supported by his group are among several that suspended operations in Syria in recent weeks as airstrikes and ground combat escalated.

“You don’t wait a week for an emergency operation and the people of Syria should not have to wait a week for relief from bombings,” Mr. Miliband said in a statement. “We wait with eager anticipation to see whether this agreement is a turning point or a false dawn.”

A senior State Department official said it was unrealistic to expect Russian airstrikes to stop over Aleppo, a rebel-held city that has been pummeled in recent days, sending civilians fleeing across the border to Turkey. Russia has insisted that other rebel groups are fighting alongside the Nusra Front, rather than against them.

The United Nations special envoy for Syria, Staffan de Mistura, said Friday that the United Nations and its humanitarian partners “will be able to reach the civilians within the coming days,” though only if they get the blessings of warring parties in those areas.

The United Nations estimates that nearly half a million people are under siege, most by the Islamic State and the Syrian government, but some watchdog groups say the number is far higher.

This is not the first time humanitarian aid has been employed as a bargaining chip in foreign policy.

During the American invasion of Afghanistan, Colin L. Powell, then secretary of state, famously described humanitarian aid agencies as a great “force multiplier.” That comment prompted sharp criticism from aid groups like Doctors Without Borders, which insisted that it was imperative for aid groups to operate with strict impartiality.

But experts say the war in Syria, having spawned the world’s greatest humanitarian crisis, has also moved the needle — backward — on politicizing aid.

The Assad government repeatedly refused permission to United Nations agencies seeking to enter Syria in order to transport aid to rebel-held areas. And the United Nations, for its part, had been reluctant to let its aid convoys cross into Syria from neighboring countries without authorization from the Security Council.

That led to months of bitter arguments among the world powers that make up the Council. Russia had backed Mr. Assad’s contention that all aid should be channeled through the government until repeated appeals by United Nations relief officials led to a deal being struck in mid-2014. Russia agreed then to a resolution authorizing cross-border convoys of emergency aid for Syrian civilians in rebel-held areas without prior approval by the authorities in Damascus.

But that did little to help those trapped behind front lines, like thousands in Madaya and neighboring Zabadani, where more than two dozen people have recently died of starvation.

David E. Sanger contributed reporting from Munich.

A version of this article appears in print on , Section A, Page 8 of the New York edition with the headline: Truce Deal Makes Aid a Political Tool, Critics Say . Order Reprints | Today’s Paper | Subscribe